Milky Way
Dana Desonie, Ph.D.
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Printed: December 7, 2014
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Milky Way
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Milky Way
• Describe the characteristics of the Milky Way Galaxy.
Have you seen the Milky Way?
There is so much light pollution in most cities that many people have never seen the Milky Way. On a clear night
away from lights, the view is of a bright white river of stars. You don’t need a telescope or even binoculars to see it.
The view of the Milky Way is so bright because you’re looking at the stars in your own galaxy.
The Milky Way Galaxy
The Milky Way Galaxy is our galaxy. Home, sweet home. The Milky Way is made of millions of stars along with
a lot of gas and dust. It looks different from other galaxies because we are looking at the main disk from within the
galaxy. Astronomers estimate that the Milky Way contains 200 billion to 400 billion stars.
Shape and Size
It is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it. Astronomers have
identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars ( Figure 1.1).
Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 lightyears across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the
disk.
The Milky Way Galaxy is a big place. If our solar system were the size of your fist, the Galaxy’s disk would still be
wider than the entire United States!
A video closeup of the Milky Way Galaxy can be seen here: http://www.space.com/common/media/video/player.p
hp?videoRef=black_holes#playerTopjjj .
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FIGURE 1.1
An artist’s rendition of what astronomers think the Milky Way Galaxy would
look like seen from above. The sun is located approximately where the
arrow points.
Where We Are
Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the
Milky Way Galaxy. Most of the stars we see in the sky are also in this spiral arm. We are about 26,000 light-years
from the center of the galaxy. We are a little more than halfway out from the center of the galaxy to the edge.
Just as Earth orbits the Sun, the solar system orbits the center of the galaxy. Astronomers have recently discovered
that at the center of the Milky Way and most other galaxies, is a supermassive black hole.
This video describes the solar system in which we live. It is located in an outer edge of the Milky Way galaxy, which
spans 100,000 light years: http://www.youtube.com/watch?v=0Rt7FevNiRc (5:10).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/1483
The Universe contains many billions of stars, and there are many billions of galaxies. Our home, the Milky Way
galaxy, is only one: http://www.youtube.com/watch?v=eRJvB3hM7K0&feature=related (5:59).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/1484
Vocabulary
• Milky Way Galaxy: Spiral galaxy in which Earth and our solar system reside.
Summary
• From inside, the Milky Way Galaxy looks like a river of stars.
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Chapter 1. Milky Way
• From outside the galaxy, the Milky Way would appear as a spiral.
• Our solar system is midway out a spiral arm of the Milky Way.
Explore More
Use the resources below to answer the questions that follow.
• The Milky Way Galaxy at http://www.windows2universe.org/the_universe/Milkyway.html
1. What type of galaxy is our galaxy?
2. Where are we located in the galaxy?
3. How old is our galaxy?
• How Far Across Is the Milky Way? at http://www.windows2universe.org/kids_space/milky_way_ask.html
4. How wide is our galaxy?
5. How long does it take our solar system to revolve around the center of our galaxy?
6. How far away is the center of our galaxy?
Review
1. Why do astronomers think that the Milky Way is a spiral galaxy?
2. Where is Earth in the Milky Way galaxy?
3. What is at the center of the Milky Way Galaxy?
References
1. Courtesy of NASA/JPL-Caltech/R. Hurt (SSC). An artistic rendition of what the Milky Way Galaxy would
look like from above. Public Domain
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